In a laser printer, printing is achieved by first scanning a digitized image onto a photoconductor. Typically, the scanning is performed with diodes, e.g. laser diodes or light emitting diodes that pulse a beam of energy onto the photoconductor. The photoconductor typically comprises a movable surface coated with a photoconductive material capable of retaining localized electrical charges. The surface of the photoconductor is conceptually divided into small units called pixels. Each pixel area is capable of being charged to a given electrical potential, but it is not completely independent of the electrical charge of each surrounding pixel due to charge sharing and the manner in which toner is attracted to the charged areas. The charge sharing effects can be utilized to create effectively larger or smaller pixels, to control the thickness of the attracted toner layer, and to reposition individual pixels horizontally or vertically by a fraction of a pixel. This is typically accomplished by using pulse-width-modulated waveforms.
The digitized image is essentially organized into a two dimensional matrix within a raster. The image is digitized into a number of lines. Each line comprises a number of discrete points. Each of the points corresponds to a pixel on the photoconductor. Each point is assigned a binary value relating information pertaining to its color and potentially other attributes, such as density. The matrix of points makes up the resultant digitally stored image. The digital image is stored in computer readable memory as a raster image. Video blocks or scan control circuitry read the raster image data and actuates the laser to selectively expose a given pixel based on the presence or absence of coloration, and the degree of coloration for the pixel. For a four-color laser printer, at least one laser scanner is included in the printer and used to generate a latent electrostatic image on the photoconductor. Generally, one latent electrostatic image is generated for each color plane, e.g. cyan, yellow, magenta, and black, to be printed.
One prior art method to provide pixel position control is use an external pulse width modulator. The horizontal justification of the pixel is limited to right, left, or center. In addition, there is increased complexity in handling horizontal synchronization.
Another prior art technique is to use a fixed tap pulse width modulator with a fixed multiple oversampling clock for pixel control. The typical pulse width modulator typically has a single pulse with justification and does not permit arbitrary waveforms. This technique further limits how the oversampling clock may be generated.
Alternatively, a feedback loop delay tap pulse width modulator can be used. The delay elements require custom layout. The design requires real-time calibration to adjust for process, voltage, temperature (PVT) and PVT drift. Delay elements require complicated production testing procedures, and delay elements are not portable. A dithered input reference cannot be used and the output frequency spectrum cannot be easily smeared to reduce radio frequency interference (RFI). Due to the complex calibration and testing features, the design is large.